In process measuring technology, e.g., in chemical, biotechnological, pharmaceutical, and food technology processes, as well as in environmental metrology, automatic analyzers are used to determine a measurand in a liquid sample. Analyzers may, for example, be used for the monitoring and optimization of the cleaning performance of a sewage treatment plant, monitoring drinking water, or monitoring quality of foods. Measured and monitored is, for example, the proportion of a certain substance, which is also called an analyte, in a sample fluid, such as a liquid or a liquid mixture, an emulsion, a suspension, a gas, or a gas mixture. Analytes may, for example, be ions, such as ammonium, phosphate, silicate or nitrate, calcium, sodium or chloride, or biological or biochemical compounds, e.g., hormones, or even micro-organisms. Other parameters that are determined using analyzers in process measuring technology, including in the field of water control, are sum parameters, such as total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), or chemical oxygen demand (COD). Analyzers may, for example, be designed as cabinet devices or buoys.
The sample to be analyzed is often treated in analyzers by mixing it with one or more reagents, thus causing a chemical reaction in the reaction mixture. The reagents are preferably selected such that the chemical reaction is verifiable by physical methods, e.g., by optical measurements, using potentiometric or amperometric sensors, or through a conductivity measurement. Using a sensing element, measured values of a measurand correlated with the analysis parameter (e.g., COD) actually to be determined are detected accordingly. The chemical reaction may, for example, cause a coloring or a change of color that can be detected using optical means. In this case, the intensity of the color is a measure of the parameter to be determined. As a measurand correlated with the parameter to be determined, an absorption or extinction of the sample treated may, for example, be determined by photometric means by feeding electromagnetic radiation, such as visible light, from a radiation source into the liquid sample, and receiving it with a suitable receiver after transmission through the liquid sample. The receiver generates a measurement signal, which depends upon the intensity of the radiation received and from which the value of the parameter to be determined can be derived for example, based upon a calibration function or a calibration table.
In order to use such methods of analysis in an automated manner, e.g., in the industrial sector or for monitoring a sewage treatment plant or a body of water outdoors, it is desirable to provide an analyzer that executes the required analytical processes in an automated manner. In addition to a sufficient measurement accuracy, the most important requirements for such an analyzer are robustness, easy operability, and the guarantee of sufficient occupational and environmental safety.
Automatic analyzers are already known from the prior art. For example, DE 102 22 822 A1, DE 102 27 032 A1, and DE 10 2009 029305 A1 show online analyzers for analyzing measuring samples. These online analyzers are respectively designed as a cabinet device, having an electronic measuring and control system, supply tanks for reagents, standard solutions and cleaning liquids, pumps for delivering and dosing the liquid sample and the reagent or reagents into a measuring cell, and a sensing element for optical measurements on the liquid sample contained in the measuring cell and converted using the reagent or reagents. The reagents, standard solutions, or cleaning liquids are taken from the supply tanks and transported into the measuring cell. Accordingly, spent liquid is transferred from the measuring cell into a waste tank.
In certain applications, it may be required to dilute the sample fluid prior to supplying it to the measuring cell for example, in order to cover a broader concentration range of the analyte. From CN 101 650 276 A is known an automatic analyzer for determining a sugar concentration in a fermentation process, in which the samples taken from the fermenter are diluted with water. This takes place by means of two pumps, the feed rates of which are determined and set by a control computer in order to set a dilution ratio.
However, this setting of the dilution ratio, i.e., the dosing of the sample fluid and of a dilution medium to be mixed with the sample fluid, by controlling two separate pumps has disadvantages: The transport of the fluids in automatic analyzers takes place via fluid lines, the respective internal volume of which can change over the service life of the devices for example, by gradual clogging of the fluid lines with a polluting load present in the sample fluid or by other formation of deposits or vegetation inside the fluid lines. If the fluid lines are made of a polymer material, their internal volume can also change as a result of aging of the polymer material for example, as a result of hardening or flowing of the polymer material. The pumps used can also be exposed to aging and material fatigue, so that the fluid volume transported using a pump with given operating parameters and given transport time also called the feed rate changes over the service life of the pump. Such signs of aging become particularly highly noticeable in the hose lines and hose pumps often used in automatic analyzers. Hose pumps also called peristaltic pumps transport the fluid to be delivered by means of external mechanical deformation of the hose lines. As a result of the mechanical loading of the hose lines, a significant change in the fluid volume delivered with given pump parameters occurs within relatively short periods of time. If the dosing of the sample fluid and the dilution medium is thus carried out solely by controlling the pumps, the actual feed rate, and thus the actually delivered fluid volume over the service life of the analyzer, changes with aging of the pumps or of the hose lines. Since it cannot be assumed that the aging of both pumps or of the respective fluid lines takes place to the same extent, the actually available dilution ratio thus also changes. This can basically be compensated for by a routine adjustment in combination with an early exchange of the fluid lines and/or the pumps. The routine performance of these measures is, however, labor-intensive and causes routine interruptions of the operation of the analyzer.